1. Field of the Invention
This invention relates to an image pickup apparatus, and, more particularly to an image pickup apparatus for a TV camera which enables the most favorable exposure when humans are the subject of the image.
2. Description of the Related Art
There is a type of image pickup apparatus for a TV camera which automatically controls exposure at the most suitable level, it being classified into two types as follows: that is, a full-plane average light measuring type, and a center-weighted light measuring (spot measuring) type.
FIG. 1 is a schematic view illustrating an essential portion of a conventional image pickup apparatus. Referring to FIG. 1, an operation will be briefly described.
Referring to FIG. 1, reference numeral 101 represents a lens, and reference numeral 102 represents a diaphragm (iris) for controlling the quantity of light. Reference numeral 103 represents an image pickup element, reference numeral 104 represents an amplifier, and reference numeral 340 represents a gate circuit. Reference numeral 330 represents an integrator, reference numeral 181 represents a comparator, reference numeral 112 represents an iris driving circuit, and reference numeral 350 represents a gate signal generator.
Incidental light from a subject, via the lens 101, reaches the iris 102 at which the quantity of light is adjusted properly, and is introduced into the image pickup element 103. Next, a signal voltage (current) in accordance with the light from the subject is obtained. This signal is amplified by the liner amplifier 104 so as to be introduced into a monitor 104" via the ensuing processing circuit 104'. The gate circuit 340 controlled by the gate signal generator 350 determines a light measuring frame of the picture plane for the output signal from the amplifier 104. Only the signal within this light measuring frame is introduced into the integrator 330 where the same is integrated. The thus-integrated signal is compared with a reference level 182 by the comparator 181, and the output therefrom controls the iris 102 via the iris driving circuit 112. At this time, if the gain of the comparator 181 is sufficiently great, the iris 102 is controlled by a controlling loop thereof in such a manner that the integrated value of the image signal within the light measuring frame becomes the same as the reference value 182. If the light measuring frame determined by the gate circuit 340 comprises the entire picture plane, the system can be called a full-plane average light measuring type, while the system can be called a center-weighted light measuring or the spot measuring type. In a case of the center-weighted light measuring method, weighting is sometimes performed so as to make the central portion of the picture plane maximum. Since such weighting of the type described above cannot be performed by a mere gate circuit shown in FIG. 1, the product of the image signal and the weight signal needs to be obtained by using, for example, a multiplier.
However, a conventional light measuring method of the type described above involves the following problems: that is, in the full-plane average light measuring, since the subject image disposed against the light exhibits great luminance in the portion against the light, the diaphragm moves to be closed if the diaphragm is controlled by an averaged signal, causing for the subject image to be darkened and hidden.
On the other hand, in a case where a brilliant subject image is disposed in front of a dark background, the diaphragm is moved in the direction to be opened since the average level of the image signal is reduced. Therefore, the subject image becomes excessively highlighted.
In a case where the center-weighted light measuring method, the most suitable exposure can be obtained when the subject image is disposed at the central portion of the picture plane. However, if the subject image deviates off the central portion, the similar phenomenon to the full-plane average light measuring occurs. That is, in a case where the subject is disposed against light, the subject becomes dark and hidden, while in a case where the same is disposed in the dark background, the subject image becomes excessively highlighted.
In particular, since a video camera of the type described above performs the signal detection in response to a luminance signal lacking a red or blue component, the diaphragm cannot be adjusted suitably if the color of the subject image disposed in the above-described specific portion is thick (in particular in a blue of red case), causing the diaphragm to be excessively opened.
As described above, in the conventional image pickup apparatus, since signal detection is performed in response to the luminance signal lacking a blue or red component, a problem arises in that the degree of diaphragm setting cannot be suitably adjusted depending upon the color of the subject image when the incidental light is diaphragm-controlled by using a specific portion of the image signal.